Structure and method for a high-speed semiconductor device having a Ge channel layer

ABSTRACT

The invention provides semiconductor structure comprising a strained Ge channel layer, and a gate dielectric disposed over the strained Ge channel layer. In one aspect of the invention, a strained Ge channel MOSFET is provided. The strained Ge channel MOSFET includes a relaxed SiGe virtual substrate with a Ge content between 50-95%, and a strained Ge channel formed on the virtual substrate. A gate structure is formed upon the strained Ge channel, whereupon a MOSFET is formed with increased performance over bulk Si. In another embodiment of the invention, a semiconductor structure comprising a relaxed Ge channel layer and a virtual substrate, wherein the relaxed Ge channel layer is disposed above the virtual substrate. In a further aspect of the invention, a relaxed Ge channel MOSFET is provided. The method includes providing a relaxed virtual substrate with a Ge composition of approximately 100% and a relaxed Ge channel formed on the virtual substrate.

PRIORITY INFORMATION

This application claims priority from provisional Ser. No. 60/299,139 filed Jun. 18, 2001.

GOVERNMENT SUPPORT

This invention was made with government support under Contract No. F19628-99-C-0043, awarded by the Air Force. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

The invention relates to the field of MOSFET fabrication, and in particular to the formation of Ge channel MOSFETs grown on SiGe/Si virtual substrates.

Channel engineering in the silicon-germanium (SiGe) materials system can result in increased electron and hole mobilities over conventional bulk Si, leading to enhanced metal-oxide-semiconductor field-effect transistor (MOSFET) performance. In particular, increases in mobility (μ) realized through channel engineering lead to increases in MOSFET drain currents and ultimately to higher switching speeds. In addition, the low hole mobility of bulk Si (μ_(hole)˜0.5 μ_(electron)) leads to increased p-MOSFET gate widths to compensate for their reduced drive currents. The increased chip area taken up by p-MOSFETs wastes valuable real estate, while the mismatch in n- and p-MOSFET areas further reduces logic speed through capacitive delays; both of which force circuit designers to avoid p-MOSFETs in logic circuits whenever possible. High mobility layers, while critical in n-MOSFETs, thus offer particularly important improvements for p-MOSFET design.

Compressively strained SiGe layers, deposited on bulk Si and capped with bulk Si to preserve the Si/SiO₂ gate interface, lead to modest increases in hole mobility, though electron mobility is unchanged. Increased process complexity and degraded short channel effects further moderate gains in circuit performance through this channel architecture. Tensile strained Si layers grown on relaxed SiGe virtual substrates offer large gains in electron and hole mobility, but the ratio of electron to hole mobility remains unbalanced. Schottky-gated modulation-doped field-effect transistors (MODFETs) incorporating buried compressively strained Ge channels on relaxed Si_(1-x)Ge_(x) (x>0.6) virtual substrates provide high hole mobility, but their limited voltage swing, high standby power consumption, and process complexity preclude their use in digital or large-scale integrated circuits. The combination of buried compressively strained Si_(1-y)Ge_(y) channels and tensile strained Si surface channels on relaxed Si_(1-x)Ge_(x) virtual substrates (y>x), hereafter referred to as dual channel heterostructures, provide high hole mobility in a complementary MOSFET (CMOS)-compatible layer structure. Peak effective hole mobilities of 760 cm²/V-s have been reported for a dual channel heterostructure p-MOSFET with a strained Si_(0.17)Ge_(0.83) channel on a relaxed Si_(0.48)Ge_(0.52) virtual substrate.

Pure Ge has the highest hole mobility of all semiconductors, along with an electron mobility comparable to bulk Si. MOSFETs based on pure Ge channels thus offer large performance gains over bulk Si. Effective mobilities as high as 1000 cm²/V-s have been reported for n- and p-MOSFETs fabricated on bulk Ge and utilizing germanium oxynitride as a gate material. However, bulk Ge substrates are not an economical manufacturing technology for integrated circuits. Also, an effective hole mobility of 430 cm²/V-s has been attained for relaxed Ge deposited directly onto a (111) Si substrate with no buffers and utilizing a SiO₂ gate. However, neither of these device structures provides the consistent control of defect density (imparted by SiGe virtual substrate technology) or well-developed gate interface (as, for example, in Si/SiO₂) required for large-scale integrated applications.

SUMMARY OF THE INVENTION

The invention provides semiconductor structure comprising a strained Ge channel layer, and a gate dielectric disposed over the strained Ge channel layer. In one aspect of the invention, a strained Ge channel MOSFET is provided. The strained Ge channel MOSFET includes a relaxed SiGe virtual substrate with a Ge content between 50-95%, and a strained Ge channel formed on the virtual substrate. A gate structure is formed upon the strained Ge channel, whereupon a MOSFET is formed with increased performance over bulk Si. In another embodiment of the invention, a semiconductor structure comprising a relaxed Ge channel layer and a virtual substrate, wherein the relaxed Ge channel layer is disposed above the virtual substrate. In a further aspect of the invention, a relaxed Ge channel MOSFET is provided. The method includes providing a relaxed virtual substrate with a Ge composition of approximately 100% and a relaxed Ge channel formed on the virtual substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section schematic of a strained Ge channel layer structure used in accordance with the invention;

FIG. 2 is a cross-section schematic of a strained Ge MOSFET in accordance with the invention;

FIG. 3 is a cross-section schematic of a relaxed Ge channel layer structure used in accordance with the invention;

FIG. 4 is a cross-section schematic of a relaxed Ge MOSFET in accordance with the invention;

FIG. 5 is a cross-section schematic of a strained or relaxed Ge channel structure on a virtual substrate comprising an insulating layer;

FIG. 6 is a cross-section schematic of a strained or relaxed Ge channel layer structure with a thin Si cap used in accordance with the invention;

FIG. 7 is a graph that demonstrates the effective hole mobilities of two strained Ge p-MOSFET devices and a bulk silicon control; and

FIG. 8 is a graph that demonstrates the hole mobility enhancement associated with two strained Ge p-MOSFET devices.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic of the layer structure upon which strained Ge channel MOSFETs are created. The layer structure includes a high quality strained Ge channel layer 14 provided on a virtual substrate 10. This strained Ge channel layer 14 may be provided on virtual substrate 10 either through epitaxial deposition or through wafer bonding techniques. In the exemplary embodiment shown in FIG. 1, the virtual substrate 10 includes a Si substrate 11, a graded composition SiGe layer 12, and a relaxed SiGe cap layer 13. The graded composition SiGe layer 12 is graded from approximately 0% Ge to a final concentration between 50% Ge and 95% Ge at a grading rate, for example, of 10% Ge/micron for a final thickness of approximately 5.0-9.5 microns. A method for providing high quality graded buffer layers is disclosed in U.S. Pat. No. 6,107,653 by Fitzgerald et al. The relaxed SiGe cap layer 13 contains 50% Ge to 95% Ge, for example, and has a thickness of 0.2-2.0 microns. A strained Ge channel layer 14 is provided on the virtual substrate 10. The strained Ge channel layer 14 has a thickness of 50 Å-500 Å and is compressively strained. The strained Ge channel layer 14 may grown at reduced temperature (T_(growth)<550° C.) to suppress strain-induced surface undulations and improve surface morphology, forming a strained Ge channel layer that is substantially planar. This planarity improves carrier mobility and facilitates device fabrication. The strained Ge channel layer 14 provides enhanced mobility and performance when it is used to create MOSFETs, while the virtual substrate 10 provides the necessary defect control and large area substrates for integrated circuit manufacturing. In a preferred embodiment, the strained Ge channel layer 14 is fabricated on the virtual substrate 10, which includes a relaxed SiGe cap layer 13 that is 70% Ge.

FIG. 2 is a cross-section of a schematic diagram of a strained Ge channel MOSFET 20 in accordance with the invention. The MOSFET 20 includes virtual substrate 10 and a strained Ge channel layer 14. A gate dielectric layer 21 is formed upon the strained Ge channel layer 14. The gate dielectric may be, for example, a dielectric comprising SiO₂ or a deposited dielectric, and possesses satisfactory integrity required for MOSFETs in operation within integrated circuits. For purposes hereof, a gate dielectric with satisfactory integrity is one that has, for example, a relatively low interface state density, e.g., less than 1×10¹¹ eV⁻¹cm⁻², and/or a relatively low leakage current, e.g., <10 nanoamperes/square micrometer (nA/μm²) to 1 microampere/square micron (μA/μm²) or even 10 μA/μm², preferably approximately 10-100 nA/μm² at 100° C. In some preferred embodiments, the leakage current may range from approximately 10-100 nA/μm². The gate dielectric thickness may be, for example 15 Å. A gate contact 22, such as doped polysilicon, is deposited on the gate dielectric layer 21. The layers are patterned by photolithography and etching. The MOSFET 20 also includes a source 23 and drain 24. The source and drain regions are defined by ion implantation. The dopant species in the source and drain is n-type or p-type for either n-MOSFET or p-MOSFET operation, respectively. By utilizing the strained Ge channel layer 14, high mobility MOSFET operation is achieved. The MOSFET 20 also includes three terminals 25, 26, and 27. The terminals 25 and 26 are used to establish electrical voltages between the source 23 and drain 24 while the terminal 27 is used to modulate the conductivity of the strained Ge channel 14 under the gate dielectric 21.

FIG. 3 is a schematic of the layer structure upon which relaxed Ge channel MOSFETs are created. The layer structure includes a high quality relaxed Ge layer 34 provided on a virtual substrate 30. This relaxed Ge layer 34 may be provided on the virtual substrate 30 either through epitaxial deposition or through wafer bonding techniques. In the exemplary embodiment shown in FIG. 3, the virtual substrate includes a Si substrate 31 and a graded composition SiGe layer 32. The graded composition layer 32 is graded to a final Ge percentage of approximately 100% at a grading rate, for example, of 10% Ge/micron for a final thickness of approximately 10 microns. The relaxed Ge channel layer 34 may have a thickness of 50 Å-2 microns.

FIG. 4 is a cross-section of a schematic diagram of a relaxed Ge channel MOSFET 40 in accordance with the invention. The MOSFET 40 includes a virtual substrate 30 and a relaxed Ge channel layer 34. A gate dielectric layer 41 is formed upon the relaxed Ge channel 34. The gate dielectric may be, for example, a dielectric comprising SiO₂ or a deposited dielectric, and possesses satisfactory integrity required for MOSFETs in operation within integrated circuits. The gate dielectric thickness may be, for example 15 Å. A gate contact 42, such as doped polysilicon, is deposited on the gate dielectric layer 41. The layers are patterned by photolithography and etching. The MOSFET 40 also includes a source 43 and drain 44. The source and drain regions are defined by ion implantation. The dopant species in the source and drain is n-type or p-type for either n-MOSFET or p-MOSFET operation, respectively. By utilizing the relaxed Ge channel layer 34, high mobility MOSFET operation is achieved. The MOSFET 40 also includes three terminals 45, 46, and 47. The terminals 45 and 46 are used to establish electrical voltages between the source 43 and drain 44 while the terminal 47 is used to modulate the conductivity of the relaxed Ge channel 34 under the gate dielectric 41.

In an alternative embodiment shown in FIG. 5, the virtual substrate 50 may comprise a Si substrate 51 and an insulating layer 52 on which the strained or relaxed Ge channel 54 is provided via wafer bonding. In the strained Ge channel case, an optional relaxed SiGe layer with a Ge concentration between 50% and 95% may also be provided between the insulating layer and the strained Ge channel layer. These structures can be provided through the layer transfer techniques disclosed in U.S. patent application Ser. No. 09/764,182.

In an alternative embodiment shown in FIG. 6, a thin Si layer 65 that may be strained or partially relaxed is provided on either the strained or relaxed Ge channel layer 64. When the Ge channel is strained, the thin Si layer may be grown at reduced temperature (T_(growth)<550° C.) initially to improve surface morphology and stabilize the compressively strained Ge channel layer against strain-induced undulations, forming a strained Ge channel layer that is substantially planar. The thin Si layer may then be grown at high temperatures (T_(growth)<400° C.) to improve the growth rate in chemical vapor deposition. The thin Si layer 65 may be initially grown upon strained or relaxed Ge channel layer at low temperatures to improve the morphology of this layer and form a thin Si layer that is substantially planar. The thickness of the thin Si layer may be minimized to reduce carrier population in this layer. The strained or relaxed Ge channel layer 64 provides enhanced mobility and performance when it is used to create MOSFETs, while the virtual substrate 60 provides the necessary defect control and large area substrates for integrated circuit manufacturing. The virtual substrate 60 may be the virtual substrates 10, 30 or 50 shown in previous embodiments. As in the embodiment discussed above with reference to FIG. 4, a gate dielectric layer 61 is formed above the Ge channel layer 64. The thin Si layer 65 provides a high quality interface between the semiconductor layer structure and the gate dielectric. A gate contact 62, such as doped polysilicon, is deposited on the gate dielectric layer 61 for modulating the conductivity of the Ge channel layer 64 under the gate dielectric 61.

FIG. 7 is a graph that demonstrates effective hole mobilities 71, 72, and 73 versus effective vertical field. Effective hole mobility 71 corresponds to a first strained Ge p-MOSFET device with a Si cap thickness of 60 Å, effective hole mobility 72 corresponds to a second strained Ge p-MOSFET device with a Si cap thickness of 50 Å, and effective hole mobility 73 corresponds to a bulk silicon control p-MOSFET. The strained-Ge channel devices exhibit a peak hole mobility of 1160 cm²/V-s.

FIG. 8 is a graph that demonstrates effective hole mobility enhancements 81 and 82 versus effective vertical field. Effective hole mobility enhancement 81 corresponds to a first strained Ge p-MOSFET device with a Si cap thickness of 60 Å, and effective hole mobility enhancement 82 corresponds to a second strained Ge p-MOSFET device with a Si cap thickness of 50 Å. At high vertical fields, FIG. 8 shows that mobility enhancement 81 is degraded as compared to mobility enhancement 82. This indicates that the holes can be pulled into the Si cap layer 65 where their mobility is not as high as in the Ge channel layer 64. The consistency of the Ge channel hole mobility enhancement 82 over a wide range of vertical electric fields demonstrates that maintaining a sufficiently low Si cap thickness (less than approximately 50 Å) allows the high field mobility enhancement to be completely preserved.

Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention. 

1. A semiconductor structure comprising: a virtual substrate comprising a compositionally graded SiGe layer having a final Ge composition ranging from about 70% to about 100%; a relaxed Ge channel layer disposed over the virtual substrate; and a gate dielectric disposed over the relaxed Ge channel layer.
 2. The structure of claim 1, further comprising: a source region disposed in a first portion of the relaxed Ge channel layer, a drain region disposed in a second portion of the relaxed Ge channel layer; and a gate contact disposed above the gate dielectric and between the source and drain regions.
 3. The structure of claim 2, wherein the source region and the drain region are p-type doped.
 4. The structure of claim 2, wherein the source region and the drain region are n-type doped.
 5. The structure of claim 1, further comprising a thin Si layer disposed over the relaxed Ge channel layer.
 6. The structure of claim 5, wherein the thin Si layer is substantially free of surface undulations.
 7. The structure of claim 1, wherein the gate dielectric is provided by thermal oxidation.
 8. The structure of claim 1, wherein the thickness of the relaxed Ge channel layer is less than approximately 2 μm.
 9. A semiconductor structure comprising: a virtual substrate; a relaxed Ge channel layer disposed over the virtual substrate; a thin Si layer disposed over the relaxed Ge channel layer; and a gate contact disposed above the thin Si layer, application of an operating voltage to the gate contact modulating movement of a plurality of charge carriers within the relaxed Ge channel layer, wherein the thickness of the thin Si layer is sufficiently small such that a majority of the plurality of charge carriers populate the relaxed Ge channel layer when the operating voltage is applied to the gate contact, thereby preserving the carrier mobility.
 10. The structure of claim 9, wherein the thin Si layer thickness is less than approximately 50 Å.
 11. A method of forming a semiconductor structure, the method comprising the steps of: providing a virtual substrate; providing a relaxed Ge channel layer disposed over the virtual substrate; and providing a gate dielectric disposed over the relaxed Ge channel layer.
 12. The method of claim 11, wherein the method further comprises the step of providing a thin Si layer disposed over the relaxed Ge channel layer.
 13. The method of claim 12, wherein the step of providing a thin Si layer comprises at least partially growing said Si layer at a temperature above approximately 400° C.
 14. The method of claim 11, wherein the method of providing a virtual substrate further comprises providing a relaxed SiGe graded composition layer.
 15. The method of claim 14, wherein the relaxed SiGe graded composition layer has a maximum Ge concentration of approximately 100%.
 16. The method of claim 11, wherein the virtual substrate comprises an insulating layer.
 17. The method of claim 12, wherein the step of providing a thin Si layer comprises at least partially growing said Si layer at a temperature below approximately 550° C. 